1. Field of the Invention
The present invention relates generally to CRTs having front panels with tensioned shadow masks affixed thereto by means of panel-mounted mask support structures. More specifically the present invention relates to speeding the exhaust cycle during manufacture of these CRTs.
2. Discussion of the Related Art
As seen in FIG. 1, a known flat tension mask (FTM) CRT envelope 11, as made by the assignee of the present invention, comprises a substantially rectangular flat glass front panel 13 and a substantially conical glass funnel 15 hermetically sealed together. The funnel 15 and panel 13 are joined by application of heat to a cementious material 17, which is a television grade devritrifying solder glass, known in the art as frit, and shown schematically in a cured, or hardened, state 18. Shadow mask support structures, or rails, 14 are affixed to the panel 13 by frit 18 and form a substantially rectangular mask-support frame 12 (FIG. 2) to support a shadow mask 16 welded thereto. Extending from the funnel 15 is a glass neck 19 into which is hermetically sealed an electron gun 21 by fusing the neck glass thereto. The envelope 11 is evacuated through a tube 23 extending through the gun 21 and the tube 23 is sealed, completing an evacuated and operational CRT. Operational components not necessary to a disclosure of the present invention have been omitted but will be understood by the artisan to be present.
In the evacuation procedure, or "exhaust cycle", the envelope 11 is hooked to vacuum plumbing (not shown) and traversed through a lehr, or oven, having sections of successively higher temperatures. The heat is required to drive contaminants inside the bulb e.g. water, into vaporous states so that they may be withdrawn from the envelope by the vacuum apparatus and a sufficient vacuum may be obtained. Heat is applied from the outside of the envelope and, therefore, a thermal gradient between the inside and outside of the envelope is established which stresses the envelope.
If the envelope is heated too rapidly during evacuation, the envelope may crack due to the stresses generated in the envelope. This envelope failure is very costly since the envelope is very nearly a completed cathode ray tube at this stage of its manufacture. In order to avoid catastrophic failure of the envelope the evacuation procedure is slowed so that the envelope is not thermally stressed at a rate higher than it can safely maintain.
In larger sized flat tension mask bulbs which utilize thicker glass in the envelope, especially in the faceplates, the thermal gradients can become more severe, thus aggravating the above-discussed failure rate versus exhaust time conditions. By attaining a desired accelerated upshock rate consistent with a low envelope failure rate and the minimum heating time needed to achieve a hard vacuum in the tube, a faster evacuation cycle with reduced envelope failure would result in manufacturing savings by reducing equipment and energy requirements while resulting in higher yields.
In past disclosures, the assignee hereof has illustrated various rail frame designs having frame corners configured to avoid contact with the funnel due to the proximity of the rail frame and funnel corners; to avoid particle contamination of the screen; and to provide inexpensive rail frames of straight ceramics which are open at the corners to avoid stress interference patterns at the rail ends which may crack the panel during rail attachment thereto.
However, until now, no reference known to the applicants has detailed the interaction of the stiff funnel corner areas with the proximate rail frame corners and suggested ways to alleviate panel stress in this area to provide faster upshock rates during envelope exhaust.